1. Field of the Invention
The present invention relates to the classification of powders of varying particle sizes, in a dynamic separator through which a gaseous stream, generally air, passes.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Powders or particles can be classified into two fractions by particle size in suspension in air, using dynamic separators. To separate the particles according to their size these separators use the forces created by the movement of the air and by the rotary separation members.
The most recent generation of separators is commonly referred to as the “third generation”. Starting from a raw product with the same particle size, these separators are able to extract more fines, in a chosen particle size range, than devices of the earlier generations.
They are provided with one or more rotors of vertical axis, fitted at their periphery with fixed selection blades, generally radial. These selection rotors, or cages, are also referred to as “squirrel cages”. The materials to be separated are powders, often of mineral origin, such as cement, lime or limestone, the particle size distribution of which may range from a few microns to a few millimeters.
The principles behind the operation of this family of separators are described, amongst others, in documents U.S. Pat. No. 4,551,241 and EP 0 023 320.
In third-generation devices like the one depicted schematically in cross section in FIG. 1, the particles are separated in a confined annular volume referred to as the “selection chamber” 3 delimited, on the outside, by the fixed vanes or louvers 5 that guide the selection air 4 and, on the inside, by the blades of the selection rotor 6. The rotor 6 is secured to a vertical shaft 8 which turns it.
The material to be selected is generally fed under gravity through several feed chutes 1 which are distributed at the upper part of the selection chamber 3. The material leaving the chutes then drops onto an annular distribution plate 2 which spins it to distribute it uniformly in the selection chamber 3. There are also separators in which the material to be classified is conveyed, to the selection chamber in suspension in the air 4, through the guide vanes 5.
Each particle entering the selection chamber 3 is subjected to the resultant of the force of gravity, centrifugal force initiated by the rotation of the turbine 6 and the drag of the selection air 4 introduced through the vanes 5. The lightest particles, referred to as the fines, enter the selection chamber 7 where they are carried by the air toward the outlet duct 9. The heaviest particles, referred to as the tails, drop under gravity into the tails chamber 10 from where they are removed, under gravity, through the tails outlet 11.
The quality of the separation is quantified by parameters taken from a curve known as the Tromp curve, which, for a given particle size fraction, indicates the quantities of tines trapped in the tails.
The cutoff point is the dimension at which any particle below this size is classified as tines and any particle above this size is classified as tails. The desired cutoff point is obtained by varying the rotational speed of the selection rotor. What happens is that increasing this rotational speed increases the centrifugal force component and therefore allows smaller particles to use centrifugal force to compensate for the drag force giving them time to drop under gravity into the tails chamber 10. This therefore reduces the cutoff diameter.
According to industry experience acquired on third-generation separators, the separation quality changes non-linearly in inverse proportion to the selection chamber concentration criterion that measures the ratio between the amount of material fed to the chamber and the flow rate of air passing through it. In other words, as the mass of particles per cubic meter of air increases, so the separation quality drops. As a result, any increase in the quality of cut can be achieved only by an appreciable reduction in this concentration. For a given flow rate of material, this reduction entails increasing the amount of air passing through the separator and therefore the volume of the selection chamber, and this increases the size of the separator and the energy consumption thereof, often undermining the profitability of the investment in relation to the commercial value of the product that is to be classified. Now, by design, a third-generator separator has only two adjustable parameters, these being the rotational speed of the rotor and, within a restricted range generally representing 10% of its nominal value, the ventilation air flow rate.
Document EP0 250 747 discloses a separator having a first separation volume, from which the tines leave directly to the outlet. The tails are conveyed to a second separation volume situated underneath, and this improves the quality of separation of the coarse tails as more fines can be removed from them. Nonetheless, this solution does not make it possible to reduce the air how rate required, but on the contrary requires this flow rate to be increased in order to feed the two separation volumes. It does not therefore allow a significant improvement in the quality of cut for a given ratio of particle mass flow rate to air volume,
Document EP0 492 062 discloses a separator with two concentric separation volumes seeking to allow the production of at least three streams of material of different dimensions, with a separation quality that is improved, although nevertheless insufficient. Such a solution does not allow a reduction in the amount of air.
Document DD 241 869 discloses a separator with two concentric separation volumes with rotors rotating in opposite directions to one another to generate a greater difference in speed and increase the flow rate of material processed with the same dimension of device. Nonetheless, such a solution does not allow an improvement in separation quality.